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ibgp default route

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  • 1.  ibgp default route

    Posted 12-24-2017 10:16

    hello,

    i hope someone can explain me why thus configuration is not working as expected:

     

    i have 2 mx with 2 upstream provider 1 for each and the mx's are connected.

     

    i have a policy that redistribute default route from ebgp to ibgp (all mx)this is the policy:

     

    term 1 {
        from {
            route-filter 0.0.0.0/0 exact;
        }
        then {
            local-preference 200;
            next-hop self;
            accept;
        }
    }
    term reject {
        then reject;
    }
    

    i have those route on the two mx:

     

    0.0.0.0/0          *[BGP/170] 2w3d 01:53:07, MED 101, localpref 200, from 185.x.x.x
                          AS path: 3xxx I, validation-state: unverified
                        > to 172.26.13.89 via xe-3/1/0.0
                        [BGP/170] 2w1d 20:32:38, localpref 100, from 217.x.x.x.x
                          AS path: 8xxx I, validation-state: unverified
                        > to 213.x.x.x via ge-1/2/2.0
    
    0.0.0.0/0          *[BGP/170] 2w4d 13:27:58, MED 101, localpref 100, from 62.x.x.x
                          AS path: 3xxx I, validation-state: unverified
                        > to 88.x.x.x via xe-0/2/0.0
    

    as you can see the 1st mx receive the default from the other one, but the first one not receive the default, infact i have checked that one mx not advertise the default to other, it is the one with the default point to other(the red one).

     

    if i create a static then ithe default appear, but i want to avoid static route, can you help understand why is not working?

    thanks



  • 2.  RE: ibgp default route

    Posted 12-24-2017 10:43
    I assume you apply this policy on one MX. Is it correct? Can you share route 0.0.0.0/0 output from both MX?


  • 3.  RE: ibgp default route

    Posted 12-24-2017 11:10
    Ok. Here’s the story. If my assumption is correct.

    When you apply import-policy on one MX router to increase local-preference for default-route to 200, it will advertise this default route to other iBGP peer (second MX).
    Now Second MX has 2 router one with default local preference 100 received from eBGP peer and another from iBGP with local preference of 200.

    Due to BGP path-selection process, second MX prefer the default route received from the iBGP peer instead of eBGP peer and install it in it’s routing-table.

    Since BGP only advertise the Best route, another mx will not advertise the default-route received from eBGP peer to iBGP because that is not the best route. This is the reason you only see one route on first MX is is correct behavior.


    HTH


  • 4.  RE: ibgp default route

     
    Posted 12-24-2017 11:24

    Based on my problem understanding as kingsman mentioned, it something as expected. Basically, you are receiving a route with higher local-preference value from the peer Mx over IPBGP Session; Which is selected in the routing table. Thus the other route in the routing table will not the active route; thus the same will not be advertised over any other bgp sessions.

     

    When a Junos device receives two similar route advertisements from different BGP peers, it will select the route will the lowest MED value. BGP route selection

    proceeds in this manner:

    Prefer the highest local-preference value

    Prefer the shortest AS-path length

    Prefer the lowest origin value

    Prefer the lowest MED value

    Prefer routes learned from an EBGP peer over an IBGP peer

    Prefer best exit from AS

    For EBGP-receive routes, prefer the current active route:

    otherwise, prefer routes from the peer with the lowest RID

    Prefer paths with the shortest cluster length

    Prefer routes from the peer with the lowest peer ID

     

     



  • 5.  RE: ibgp default route
    Best Answer

    Posted 12-24-2017 20:53

    Hello there,


    @gullio23 wrote:

    you can see the 1st mx receive the default from the other one, but the first one not receive the default,


    This is expected. You will need to add this command to make both MX to adertise eBGP route to each other even if iBGP route is best:

    https://www.juniper.net/documentation/en_US/junos/topics/reference/configuration-statement/advertise-external-edit-protocols-bgp.html

    HTH

    Thx
    Alex



  • 6.  RE: ibgp default route

    Posted 12-25-2017 01:50

    Hello everyone,

     

    Thanks a lot for explanation i have already think so but i cannot find the solution, the advertise external was the correct solution infact now i see the default correctly on other mx.

     

    Thanks a lot ana merry xmas